Generally, in a welding apparatus of stainless steel pipe and a welding method of the same, a stainless steel plate with a predetermined thickness is rolled in a circular shape, and a welding material is fused to weld a V-shaped gap formed between joining portions at both ends of the steel plate rolled in a circular shape so as to continuously produce a stainless steel pipe. When the steel pipe has a small thickness, base metal is fused to perform welding.
FIG. 1 is a perspective view of a conventional welding apparatus of a stainless steel pipe. FIG. 2 is a cross-sectional view of the conventional welding apparatus of FIG. 2. Referring to FIGS. 1 and 2, the conventional welding apparatus generates arc from an electrode rod 5a of a plasma or TIG (Tungsten Inert Gas) welding torch 5 so as to fuse base metal or a welding material 3 positioned at a gap 1a between joining portions of a steel pipe 1 rolled in a circular shape. Then, the base metal or welding material 3 is deposited into the gap 1a to form a welding bead portion 3a, thereby completing the formation of pipe.
In this case, in order to prevent the welded portion from coming in contact with oxygen and being then oxidized, inert argon gas stored in a gas storage container (not shown) is jetted onto the surface of the welding bead portion through the inner space 5b of the TIG welding torch 5 and is also jetted onto the inner surface of a welding bead portion positioned inside the steel pipe 1 through a gas supply pipe 7, as shown in FIG. 2. Then, the welding bead portions 3a inside and outside the steel pipe are shielded from the air so as not to come in contact with oxygen, and thus the welding bead portions are prevented from being oxidized.
Further, while the steel pipe 1 is transferred in a state where the welding torch 5 and the gas supply pipe 7 are fixed, or while the welding torch 5 and the gas supply pipe 7 are transferred in a state where the steel pipe 1 is fixed, the steel pipe 1 is continuously welded.
Meanwhile, the position where inert argon gas is jetted onto the welding bead portion 3a′ inside the steel pipe by the gas supply pipe 7 is located right under the portion of the welding torch 5 where arc is generated. Therefore, when a jetted amount of argon gas is increased so that the welding bead portion fused at a temperature of more than 1,350° C. is rapidly cooled, a concave groove is formed around the inner welded portion by a jetting force of the argon gas, as shown in FIG. 3. Further, the base metal is cooled so that the deposition of fused metal is reduced, and welding stress occurring in the welding bead portion and welding-heat-influenced portion is not removed. In addition, intergranular corrosion is promoted in the welding bead portion and the welding-heat-influenced portion such that carbide is educed.
As for a conventional welding method of stainless steel pipe, there is provided a method in which inert argon gas is widely jetted across the front and rear side of a portion of a welding torch where arc is generated. In this method, however, the above-described problems occur.
Meanwhile, as for work-hardening or welding-stress annealing temperature of austenitic stainless steel which is commonly used for manufacturing a stainless steel pipe, 1,050° C. is proper. The temperature range of sensitization where carbide promoting intergranular corrosion is educed is 425 to 870° C. Therefore, in order to perform annealing after a steel pipe is manufactured, the steel pipe should be heated up to a temperature of 1,050 to 1,150° C. and should be then cooled down to a temperature of less than 425° C. In this case, when plasma or TIG welding is performed, the fusing temperature of a welding material typically ranges from 1,350 to 1,500° C. However, the welding bead portion and the welding-heat-influenced portion is slowly cooled after the welding. Therefore, welding stress is not removed, and carbide is educed.
When the hardness of the steel pipe welded according to the conventional welding method is measured, the hardness of the welding bead portion and the welding-heat-influenced portion is higher (as much as 50-100%) than that of base metal. Therefore, because of a metal structure divided into an austenitic structure and a martensitic structure, corrosion caused by a potential difference inevitably occurs.
When an amount of argon gas jetted onto the welding bead portion 3a′ inside the steel pipe, positioned right under the arc generation position of the welding torch 5, from the gas supply pipe 7 is increased more than two times, relatively high-pressure and high-temperature argon gas is directly jetted onto the welding bead portion 3a′, which is still fused, such that a dent portion B is formed toward the outer surface of the still pipe, as shown in FIG. 3, because the temperature of the argon gas, which is evaporated from liquefied gas through an evaporator and jetted, is as low as 10° C. Further, since the welding bead portion is rapidly cooled in a state where the base metal is not sufficiently deposited, imperfect welding occurs, and cracks occur in the welded portion in a severe case.